Conventional cooking is typically accomplished by placing the food of choice either on, under or within a hot environment.  Whether in an oven or on a stovetop, under a boiler, or on a grill, heat is conducted from the external source to the surface of the food and, from there, it slowly penetrates to the food's interior.  Thus, in a conventional oven, the air surrounding the food must be heated first, before it can begin to cook the food.  Then, once the outer layer of food becomes heated and begins to cook, the heat can gradually penetrate to the center of the food by conduction.

Microwave ovens contain a magnetron tube, which converts electricity into microwaves rather than heat.  Microwaves are HIGH-frequency electromagnetic waves similar to radio waves.  When, early in the history of microwave ovens, detractors pointed out that these ovens cooked by radiation, skeptics mistakenly declared that microwave radiation was similar to the energy released by a nuclear bomb.  Thus the expression (and early-on misconception) that people were “nuking” their food - and even that they might someday start glowing in the dark if they microwaved their food.  This is because they believed that microwave radiation, like atomic radiation, remained in their oven - and, in their food.  However, since microwaves are similar to radio waves, there is no residual radiation - microwaves are simply converted to heat when they contact food.

With a microwave oven, it is unnecessary to preheat the interior of the cooking cavity.  Rather, the microwave energy actually penetrates the food (passing straight through the surrounding air space), thus reducing cooking time and saving electricity.  Microwaves themselves are not hot, but are attracted to and absorbed by water, fat, salt and sugar in the food.  The microwaves cause the water, fat, salt or sugar molecules in the food to rotate rapidly (at a rate of up to 2½ billion times per second) and bump into each other, causing the food to heat by friction of the food’s molecules.  This is a process not dissimilar to what happens when someone rubs their hands together and their hands quickly get warm.

Though microwaves don’t actually cook food, the friction resulting from the vibrating molecules produce the heat which does the cooking.  Microwaves do not actually touch most of the food they cook.  However, the heat generated by the vibrating molecules on the outer edges "conduct" this heat inward, layer by layer, to cook the food. Thus, the cooking of larger, more dense foods begins on the edges and heat is conducted to the center to cook the rest of the food..

Stirring helps food to cook more uniformly as it redistributes heat from the outer layers of the food to the inner layers.  The manner in which the food is arranged, the dish style (round vs. square), the density of the food, etc., all play important roles in microwave cooking.

In microwave cooking, the speed of cooking is determined by the power level selected. Many foods cook successfully at 100% (full power).  However, since personal tastes vary widely, if the cooking result is not optimized to your liking, the next time you cook that particular food, try selecting a lower power level and adjusting the cooking time appropriately - the slightly longer cooking time is often well worth it, in exchange for the results desired.  For example, foods such as casseroles benefit from longer, slower cooking times, so thus are cooked at a lower power level (just like adjusting the temperature down in a conventional oven).

Since microwaves cannot penetrate through metal, they remain completely contained within the cooker cavity (though the holes in the door panel allow you to see inside the oven during cooking, they are not large enough to allow the microwaves to pass through to the outside of the microwave).  Although microwaves do not directly heat cookware, heat building up in food can be transferred to its dish.  So containers can become very hot.  Always use potholders when removing dishes from the microwave.